1. Field
The embodiments discussed herein are directed to a polarization controller and a polarization mode dispersion compensator.
2. Description of the Related Art
In recent years, along with the development of faster optical communication technologies, the transition rates are increasing, e.g., from 10 Gbit/s systems to 40 Gbit/s systems has been in progress. Therefore, it is important to develop an optical device used in a communication system having a transmission rate exceeding 40 Gbit/s.
An increase in the transmission rate in optical communication causes an issue of degradation of the signal waveform due to the polarization mode dispersion (PMD). The polarization mode dispersion refers to the phenomenon of a delay occurring between two mutually perpendicular modes in an optical fiber (DGD: Differential Group Delay) due to a birefringence factor attributed to the ovalization of the core or the internal stress in the optical fiber. In general, the polarization mode dispersion in the optical fiber is proportional to the square root of the distance. Further, the allowable value of the polarization mode dispersion is inversely proportional to the bit rate. In long-distance high-speed transmission, therefore, a function of compensating for the polarization mode dispersion is required.
As a configuration for compensating for the polarization mode dispersion, a PMD compensator has been proposed which includes a polarization controller and a DGD compensator. This configuration compensates for the first-order polarization mode dispersion, but is incapable of compensating for the second- and higher-order polarization mode dispersion (e.g., Japanese Laid-open Patent Publication No. 2005-260370).
As another configuration for compensating for the polarization mode dispersion, a polarization mode dispersion compensator has been proposed which includes a compensation unit including a first polarization controller and a group delay time difference providing unit, a second polarization controller, and a control unit. The group delay time difference providing unit provides a group delay time difference to the light output from the first polarization controller. The second polarization controller converts the state of polarization of the light output from the compensation unit into linear polarization. The control unit controls the compensation unit and the second polarization controller to minimize the intensity of the polarized light perpendicular to the linearly polarized light output from the second polarization controller. However, even this configuration is unable to completely compensate for the second-order polarization mode dispersion (e.g., International Publication Pamphlet No. WO 2004/013992).
As still another configuration for compensating for the polarization mode dispersion, a PMD compensator has been proposed which includes a demultiplexer and a liquid crystal modulation device. The liquid crystal modulation device separately controls the respective polarizations of frequency components. Therefore, the first-order polarization mode dispersion and higher-order polarization mode dispersion are appropriately controlled. In general, however, a liquid crystal modulation device has a low response speed. If the state of polarization in a transmission line quickly fluctuates, therefore, the polarization mode dispersion is not sufficiently compensated (e.g., H. Miao et al., “All-Order PMD Compensation via VIPA Based Pulse Shaper,” OFC2008 OThG2 2008).
As the related art, the following patent documents and non-patent documents have also been proposed: Japanese National Publication of International Patent Application No. 2005-502265; International Publication Pamphlet No. WO 2004/029699; H. Miao et al., “Feed-Forward Polarization-Mode Dispersion Compensation with Four Fixed Differential Group Delay Elements,” IEEE Photonics Technology Letters Vol. 16, No. 4, April 2004, pages 1056 to 1058; and C. Xie et al., “Dynamic Performance and Speed Requirement of Polarization Mode Dispersion Compensators,” Journal of Lightwave Technology, Vol. 24, No. 11, November 2006, pages 3968 to 3975.
However, conventional techniques are unable to sufficiently compensate for high-order polarization mode dispersion, for example, if the state of polarization quickly changes.